1. Field of the Invention
The present invention relates to stereolithographic apparatus and method for optically manufacturing a three-dimensional object by using photohardenable resin, and particularly the present invention relates to stereolithographic apparatus and method for exposing the overall surface of a photohardenable resin layer to light through a mask at a time to optically manufacture a three-dimensional object, and also to stereolithographic apparatus and method for optically manufacturing a three-dimensional object having a complicated shape such as an overhang portion, portions which are separately mounted, plural leg portions which are different in length, or the like by using specific photohardenable resin composition. In the following description, “stereolithographic process” is defined as a process of exposing photohardenable resin or photohardenable resin composition to light to form a photohardened layer, and repeating the light exposing operation on photohardenable resin (composition) to laminate photohardened layers on a layer basis, thereby optically forming a desired three-dimensional object.
2. Description of the Related Art
In general, liquid photohardenable resin composition (hereinafter referred to as “photohardenable resin”) has been widely used as coating (particularly, hard coating), photoresist, dental materials, etc. Recently, much attention has been paid to a so-called stereolithography technique which optically forms a three-dimensional object on the basis of data output from a controller such as a three-dimensional CAD system or the like by using photohardenable resin because a three-dimensional object can be optically formed in desired shape and size with high precision even when it has a complicated structure. With respect to the stereolithography technique, Japanese Laid-open Patent Application No. Sho-56-144478 discloses a stereolithographic method for repeating a process of applying a required amount of optical energy to liquid photohardenable resin under control to harden the photohardenable resin as a thin layer, further supplying liquid photohardenable resin onto the hardened resin layer and then exposing the liquid photohardenable resin to light controlled on the basis of stereolithographic data from a controller to harden the liquid photohardenable resin and laminate the thin hardened photohardenable resin layer on the preceding hardened resin layer, whereby a subsequent hardened photohardenable resin layer is successively laminated on a preceding hardened photohardenable resin layer to manufacture a desired three-dimensional object. Further, a practical use method of the stereolithographic method disclosed in the above publication is proposed in Japanese Laid-open Patent Application No. Sho-60-247515, and then various proposals on the stereolithography technique have been made.
As a method of optically manufacturing a three-dimensional object has been generally and widely used a method of selectively irradiating laser beams such as ultraviolet laser beams or the like to the liquid surface of liquid photohardenable resin put in a stereolithographic bath under the control of a computer to harden the photohardenable resin so that a photohardened resin layer having a predetermined thickness and a desired pattern is obtained, then supplying a layer of liquid photohardenable resin onto the photohardened resin layer and then likewise exposing a laser beam such as an ultraviolet laser beam or the like to the liquid photohardenable resin layer to harden the photohardenable resin layer, and repeating the lamination/photohardening operations until a targeted three-dimensional object is obtained.
In general, it takes a long time to irradiate laser beams to a layer of photohardenable resin until the photohardenable resin layer is hardened, and for the purpose of increasing the stereolithographic process speed, an apparatus for forming a mask and irradiating the overall surface of a photohardenable resin layer through the mask pattern by an ultraviolet lamp at a time (hereinafter referred to as “plane-exposure”) has been proposed.
According to such a plane-exposing apparatus, a mask having a predetermined pattern formed on the surface thereof is formed and superposed on a unhardened photohardenable resin layer, and then the overall surface of the unhardened photohardenable resin layer is exposed to ultraviolet rays through the mask at a time (i.e, plane-exposed), thereby hardening the photohardenable resin layer in accordance with the mask pattern.
In the plane-exposing apparatus, however, since the mask is not brought into close contact with the unhardened photohardenable resin layer in the exposure process, uneven portions are formed on the surface of the hardened resin layer and thus it is required to cut out these uneven portions layer by layer after the hardening operation of the photohardenable resin is completed.
Furthermore, in a process of forming a photohardenable resin layer, a solid surrounding member is beforehand formed and fixed so as to surround the photohardenable resin layer, and then unhardened photohardenable resin is supplied into the inside of the fixed solid surrounding member. Therefore, unhardened photohardenable resin remains in the solid surrounding member. If the uneven portions on the surface of the hardened resin layer are cut out while the residual unhardened photohardenable resin is left, the corner portions of the hardened resin layer may be defected. In order to avoid this disadvantage, after the photohardenable resin layer is hardened, the unhardened photohardenable resin is scraped up, wax is filled into the scraped portions to prevent defects and then the uneven portions on the surface of the hardened resin layer are cut out. Therefore, extra wax, etc. are required.
Still furthermore, three-dimensional objects having complicated shapes such as overhang portions, separately-mounted portions, plural leg portions which are different in length, uneven portions, etc. have been widely manufactured by using the conventional stereolithography technique. Individual hardened layers which are successively formed by light irradiation are extremely thin, and thus a laminate obtained by laminating these thin layers is also thin. Therefore, the laminate thus finally obtained has a lower shape holding performance. In addition, photohardenable resin in a stereolithographic bath is liquid, and it has little capability of supporting a photohardened layer. Therefore, when a three-dimensional object having a complicated shape such as overhand portions, separately mounted portions, leg portions which are different in length, uneven portions or the like is manufactured, there is liable to occur such problems as hang-down, deformation, dimensional deviation, positional shift, etc. of stereolithographically-formed sites formed by photohardening during the stereolithographic process. Accordingly, in order to avoid these problems, there has been generally adopted a method of disposing a separately-formed support in a stereolithographic bath and stereolithographically forming a desired three-dimensional object while the object being formed is supported by the support (hereinafter referred to as “support basing method”), or a method of stereolithographically forming a desired three-dimensional object while an extra support portion is simultaneously formed together with the desired three-dimensional object (hereinafter referred to as “support attaching method”).
In the following description, the support-based supporting method and the support-attached supporting method will be described in detail with reference to FIGS. 1 to 4F particularly when these methods are applied to the stereolithographic process of forming three-dimensional objects having specific structures.
In the case of a three-dimensional object having a disc portion 202 between upper and lower cylindrical portions 201a, 201b as shown in FIG. 1, when the stereolithographic operation is carried out on a layer basis from the lower end of the cylindrical portion 201a, overhand portions are formed at the disc portion 202 because the diameter of the disc portion 202 is larger than that of the lower cylindrical portion 201a. 
In this case, if the stereolithographic operation is carried out on the overhand portions without using any support, there would occur such a problem that the disc portion 202 is formed so as to hang down or obliquely extend during the stereolithographic operation, and thus it is difficult to design the disc portion in a horizontal disc structure. Therefore, it is difficult to manufacture a three-dimensional object having desired shape and dimension.
In order to solve this problem, a separate support 203 as shown in FIGS. 2A and 2B is disposed in a stereolithographic bath and the stereolithographic operation is carried out while the overhand portions are supported by the support 203 (the support basing method). Here, FIG. 2A is a longitudinal sectional view of a desired three-dimensional object supported by a support when the stereolithographic operation is carried out, and FIG. 2B is a plan view taken from the lower side of the stereolithographically formed object.
Alternatively, in order to solve the above problem, the stereolithographic operation is carried out while a support portion 204 as shown in FIGS. 3A, 3B is formed integrally with the desired three-dimensional object to prevent the overhand portions from hanging down or being deformed (the support attaching method). Here, FIG. 3A is a longitudinal sectional view showing a desired three-dimensional object and a support which is integrally formed with the object, and FIG. 3B is a plan view taken from the lower side of the stereolithographically formed object.
Further, in the case of a three-dimensional object having a central joint plate portion 205, right and left leg portions 206 and 207 which are different in length and extend downwardly from the central joint plate portion 205, and right and left arm portions 208 and 209 extending upwardly from the central joint plate portion 205 as shown in FIG. 4A, the stereolithographic operation is started from the lower end of the longer leg portion 206 in a stereolithographic bath 212 in which liquid photohardenable resin is put as shown in FIG. 4B, and at the time when the height of the leg portion 206 reaches the position corresponding to the lower end of the shorter leg portion 207 as shown in FIG. 4C, the stereolithographic operation is carried out on both the light and left leg portions 206 and 207 at the same time. In this case, a thin-layer portion (photohardened resin layer) 207a constituting the shorter leg portion 207 is not joined to a thin-layer portion (photohardened resin layer) 206a constituting the longer leg portion 206. In addition, the thin-layer portion 207a is not mounted on a mount table, but floated on the liquid photohardenable resin, so that it is liable to be moved. Therefore, the distance between the thin-layer portions 206a and 207a cannot be kept to the proper value in design.
Therefore, in order to avoid this disadvantage, the conventional stereolithographic technique has generally used a method in which as shown in FIG. 4D (longitudinal sectional view) and FIG. 4E (plan view taken from the upper side), a support portion 210 for joining the thin-layer portion 207a to the thin-layer portion 206a is formed simultaneously with the start of the stereolithographic formation of the thin-layer portion 207a of the shorter leg portion 207 (i.e., the support attaching operation is carried out), thereby manufacturing a three-dimensional object having the support portion 210 as shown in FIG. 4F.
In the case of the support basing method, there is required a cumbersome work for forming a support in advance and disposing it in a stereolithographic bath. In addition, it is required that the shape and dimension of a support which is suitable to smoothly prevent hang-down and deformation at an overhang portion are designed in advance and the support is disposed at a proper position, so that great skill is required for the design and use of the support. Further, a support contact mark is liable to remain at a portion on the surface of the three-dimensional object thus formed at which the object was supported by the support. Therefore, the three-dimensional object thus formed has a defective appearance, and a repair treatment of polishing and smoothening the defective portion is required if occasion demands.
In the case of the support attaching method, there is required a cumbersome work for cutting the support portion integrally formed with the three-dimensional object after the stereolithographic, operation is completed, thereby removing the undesired support portion. In addition, in order to prevent the appearance of the three-dimensional object from being defective due to the cutting of the support portion, it is necessary to take the shape, size and mount position of the support portion into sufficient consideration, so that sufficiently great skill is required for the support attaching method. Further, when the support portion is removed, it is necessary to remove the support portion while the portion serving as the support portion in the three-dimensional object is sufficiently discretely discriminated from the other portions constituting the desired three-dimensional object (target object). Therefore, if a worker is not skilled to the extent that he/she can understand CAD data, drawings of parts, etc., it is more difficult to perform the removing work of the support portion.
In order to solve the above problems of the conventional stereolithography technique, Japanese Laid-open Patent Application No. Sho.-63-72525 discloses a method in which solidifying material such as wax or the like is used as a second material together with a first material of liquid photohardenable resin and a three-dimensional object is optically (stereolithographically) formed while hang-down and deformation at overhand portions, etc. are prevented by the solidifying material
According to this method, the solidifying second material functions as a support material for supporting a thin layer of photohardened photohardenable resin to prevent the hang-down, the deformation, etc. at the overhang portions, etc. However, in the stereolithographic method disclosed in the above publication, the second material for supporting the shape of an object being stereolithographically formed is separately required together with photohardenable resin with which a desired three-dimensional object is formed. In addition, use of the second material adds the normal process with a step of sucking unhardened photohardenable resin after the light irradiation step is carried out. Further, it further adds many other steps such as a step of coating the solidifying second material in empty portions occurring due to the suction of the unhardened photohardenable resin, a step of polishing and flattening the upper surface of the solidified second material to enhance the dimensional precision in height direction, etc. As a result, the stereolithographic work is extremely complicated, and much labor and time are required to complete the stereolithographic process, so that the stereolithographic apparatus is complicated in construction, scaled up in size and increased in price.
Furthermore, if the second material such as wax or the like coated on the photohardened layer is not sufficiently removed in the flattening and polishing step and thus remains on the photohardened layer, the second material such as wax or the like would be interposed between the photohardened layer and a next photohardened layer laminated thereon, so that the adhesion between the photohardenable layers is disturbed, the laminated photohardenable layers are liable to be peeled off each other and the strength of the three-dimensional object thus formed is lowered.
Still furthermore, photohardenable resin used in the stereolithography technique is generally expensive, and thus unhardened photohardenable resin which has not been photohardened (subjected to the photohardening treatment) has been generally withdrawn and reused after the three-dimensional object is manufactured. However, in the above stereolithographic method disclosed in the above publication, a lot of the second material such as wax or the like is contaminated in unhardened photohardenable resin, and thus it is required to reuse the unhardened photohardenable resin for the stereolithographic process after the second material is perfectly removed from the unhardened photohardenable resin. Therefore, much labor and much cost are needed to purify, withdraw and reuse the photohardenable resin.